Generally, an image sensor is a semiconductor device for converting an optical image into an electric signal. There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices, hybrid focal plane arrays, etc. The various types of image sensors may be broadly categorized as charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) image sensors.
In recent years, there has been increased interest in CMOS imagers for possible use as low cost imaging devices. CMOS image sensors are now used in many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, star trackers, motion detection systems, image stabilization systems and high-definition television imaging devices.
In a conventional CMOS pixel, an electrical signal representing input light brightness is converted to a corresponding electrical signal by, for example, a pinned photodiode. Readout circuitry couples the converted electrical signal in the form of an output voltage from a row transistor to an output terminal. In applications, a column of CMOS pixels may be coupled to the same output terminal. By selectively applying a row address signal to the gate of a selected row transistor, a selected one of the CMOS pixels may be coupled to the output terminal.
A prior art method for converting an analog signal present on a column out to a digital signal for use by digital imager circuitry using A/D conversion requires two ADCs per column to obtain a large dynamic range per column. There is a high gain channel provided to an input of a first ADC and low gain channel provided to an input of a second ADC. The high gain channel is relatively immune to noise but saturates the first ADC at a relative low input signal level. The low gain channel provides for a much larger input signal to the second ADC but is subject to a relatively high referred electron noise. The two ADC outputs may be spliced to form a single data signal with fewer bits having low noise and large dynamic range.
For example, the high gain ADC provides a saturation level of 1000 e/pixel (i.e., charges per pixel) relative to a pixel full signal of 25,000 e/pixel. The second ADC has low front end gain and provides for an input signal of up to 25,000 e/pixel at saturation but has relatively high noise because of low front end gain. The two spliced ADC output form a single digital signal having noise of about 2 e/pixel (rms) and a full signal of 25,000 e/pixel. When employing this ADC architecture, the pixel itself needs to provide an output signal over this full dynamic range.
For imagers that operate at relatively high data rates, such as one having 5 Megapixels and configured to operate at 100 fps, typically 8 to 16 digital output ports may be required. For the dual ADC per column approach using 11-bit ADCs, about 174 extra bonds pads and package pins are required. This results in higher packaging costs, greater camera complexity and higher on-chip power dissipation. Increased chip power dissipation is undesirable because it may result in a higher dark, resulting in an imager that has reduced sensitivity or a need for increased cooling.
Accordingly, what would be desirable, but has not yet been provided, is CMOS imager having a single ADC per column capable of achieving both low noise and a large full signal (dynamic range).